Wycombe:
It's done using what's called a "Level 2 Variation" which is a set of conditions that allow an extended FDP, as
Plumponpies says above.
Amongst other things it requires the crew to be off-duty by a certain time the previous day and to be rostered a "Recovery Day" free of all duties afterwards. There are other aspects to it too, but that's the basics.
It's not usually popular though!
The new BA 777 service started while I was out there, and I noticed it has a rather longer stop (approx 5 hrs) in SSH - is that for some sort of rest?
Well, they could get a little bit of rest if they were using a split duty, but you wouldn't get much useful FDP extension (if any) with only a 5 hour stopover. They probably slip crews downroute I would guess. I don't know though as I don't work for BA.